DESCRIPTION (Investigator's Abstract): "Rationale: Retinopathy of Prematurity (ROP) remains a serious health problem causing significant visual sequelae in about 3,400 premature survivors per year in the United States, despite the now recognized use of Cryotherapy. Case control studies in premature infants, anecdotal reports of successful treatment of ROP with oxygen, and studies conducted in an animal model demonstrate that chronic (weeks-long) hypoxia induced by breathing 13 percent inspired oxygen (mean PAO2=38) during the model's healing process may be a critical factor associated with and perhaps influencing cases that progress rather than regress. This hypothesis is supported by the association such as diabetic or sickle cell retinopathy.These data support the hypothesis that: 'Supplemental Therapeutic Oxygen for Prethreshold ROP (STOP-ROP) will reduce, by at least one-third, the number of infants with one or both eyes progressing to Threshold ROP.' Infants who develop Prethreshold ROP will receive continuous pulse oximetry saturation monitoring and be randomly assigned to oxygen administration at one of two specified oxygen levels, conventional vs supplemental. Their ROP status will be measured and recorded prospectively, and the primary outcome variable will be the proportion of infants who progress to Threshold ROP in at least one eye by 3 months after their expected date of full term delivery. The projected sample sizes are compatible with a 2-3 year enrollment using 15-25 centers, the study is planned to coordinate efforts for data management, budget, personnel, study forms, and the DSMC with another, complimentary study of light reduction to prevent ROP (LIGHT-ROP). Follow-up will continue to the age of 3 months after each infant's original due date. Outcomes will include the number of infants who require cryotherapy or laser, ophthalmic outcomes, growth, pulmonary status, and length of hospital stay."